1. Field of the Invention
The present invention relates to a semiconductor device and, more particularly, it relates to a method and an apparatus for manufacturing a T-shaped control electrode for a field effect semiconductor device.
2. Description of the Prior Art
As semiconductor technology has matured, it has become necessary to speed up the operating functions of a semiconductor device. In a semiconductor device operating at a very high speed, it is necessary to employ an element structure such that the distance of electron movement in the semiconductor material is as short as possible. A typical example of such a semiconductor device is a field effect semiconductor device employing a T-shaped control electrode.
A conventional method of forming such a T-shaped control electrode is described with reference to FIGS. 1A to 1E.
As shown in FIG. 1A, a low-sensitivity positive resist film 2 of about 0.2 .mu.m in thickness is formed on a substrate 1 and a high-sensitivity positive resist film 3 of about 0.5 .mu.m in thickness is formed on the low-sensitivity positive resist film 2. The double-layer resist films 2 and 3 are treated by the same developing solution.
Then, electron beams 4 are applied to desired positions as shown in FIG. 1B, to expose the high-sensitivity positive resist film 3 and the low-sensitivity positive resist film 2.
After the exposure, the films 2 and 3 are subjected to development. While the upper high-sensitivity positive resist film 3 is readily soluble in the developing solution, the lower low-sensitivity positive resist film 2 is not easily dissolved as compared with the high-sensitivity positive resist film 3 with respect to the same exposure of electron beams. Thus, a resist pattern of the configuration as shown in FIG. 1C is formed by openings 5 and 6 through such difference in solubility.
Then the entire substance including the resist pattern portion is subjected to vacuum evaporation, thereby to form a control electrode film 7 of about 0.6 .mu.m in thickness as shown in FIG. 1D.
Finally, the entire substance is dipped in a solution which can dissolve the high-sensitivity positive resist film 3 and the low-sensitivity positive resist film 2 to also remove the control electrode film 7 located on the high-sensitivity positive resist film 3, thereby to obtain a T-shaped control electrode 8 as shown in FIG. 1E.
Methods of forming such T-shaped control electrodes through double-layer positive resist films of different sensitivity are disclosed in "Double-Layer Resist Films for Submicrometer Electron-Beam Lithography" (Y. Todokoro), 1980, IEEE Vol. ED-27, No. 8, pp. 1443-1448; "Formation of a T-Shaped Gate through Electron Beam Exposure" (Y. Todokoro et al.), Japan Society of Applied Physics, 1979 (Autumn), 2P-A-4; "Submicrometre Lift-Off Line with T-shaped Cross-Sectional Form" (H. Matsumura et al.), 1981 Electronics Letters, Vol. 17, No. 12, pp. 429-430; and "Method of Forming a T-shaped Gate Pattern" (Y. Yamashita), Japan Society of Applied Physics, 1984 (Autumn).
However, although the conventional method of forming a T-shaped contorl electrode required two types of resist films of different sensitivity which are treated by the same developing solution, a mixing action may take place in a boundary portion between the resist films since the components of the resist films are substantially identical to each other. The difference in sensitivity is indistinct in the vicinity of the boundary portion, and hence it is difficult to form an accurate resist pattern, i.e., a T-shaped control electrode of high accuracy after development.
Further, irradiation of electron beams must be executed in sequence per desired pattern through electron beam exposure technique, whereby the throughput is reduced.
"Synchrotron Lithography for Sub-Half-Micron T-Gates in GaAs-FETs" (K. H. Muller et al.), in the 1986 "Microcircuit Engineering" discloses a method of laminating three resist films of different sensitivity and exposing the same by X-rays thereby to form a T-shaped control electrode as a resist pattern of a desired sectional configuration.
"A Half-Micron Gate GaAs FET Fabricated by Chemical Dry Etching" (S. Takahashi et al.) in the Japanese Journal of Applied Physics, 1977, Vol. 16, Supplement 16-1, pp. 115-118 discloses a method of forming two layers of dissimilar metal films on a semiconductor substrate thereby to form a T-shaped control electrode through exposure and chemical etching.
Further, Japanese Patent Laying-Open Gazette No. 31135/1982, entitled "Method of Batch Formation of a Metal Pattern" (A. Ozawa et al.), discloses a method of batch formation of a metal pattern having different two stages for serving as an absorber of an X-ray mask comprising an absorber having double contrast.
In this method, a structure film formed by laminating two types of resist films of different sensitivity is exposed to form a resist pattern having two different stages. This resist pattern is employed to transfer the pattern to an intermediate film on a substrate provided under the same, thereby to form a desired metal pattern on the substrate by employing the intermediate film as a model.
However, none of the aforementioned prior art examles provides the advantages of the method this invention for forming a T-shaped control electrode of high accuracy and high throughput. Further, the conventional method of forming an X-ray mask having double contrast requires a laminated resist film and an intermediate film and includes a complicated step of transferring a pattern from a resist pattern to the intermediate film, and hence this method is leads to a more difficult process.